The Vltava River at Prague’s Mánes exhibition hall turned blood-red on Monday afternoon, after activists from an art group calling themselves Bolt958 poured an unknown substance into the water. The group charged that the famous art gallery “was bleeding”. The protest was over how the space, in their view, is being misused.

The Mánes Exhibition Hall is one of Prague’s best known art galleries, with a history dating back to the 1920s. In past years, the hall, run by the Manes Union of Fine Arts, featured well-known artists such as Petr Nikl or Milan Knížák. The river around gallery has also regularly features floating installations as part of the annual Sculpture Line, which is the case even now – a sculpture of a giant octopus by artist Paluš Viktor.

But it was something different that was floating on the surface of the Vltava on Monday, after activists form a group calling itself Bolt958 criticized the management of gallery for, in their view, too often leasing the space for commercial presentations and non-artistic purposes. They charged on Monday that “Mánes is bleeding” – pouring buckets of blood-red liquid into the water. Soon, red water stretched all the way from Mánes to the nearby National Theatre. The police were called in to investigate while fire fighters checked the liquid. Martin Kavka is the spokesman for Prague’s fire brigade.

“We were able to determine that the substance was not on the list of dangerous substances. In all likelihood, an inorganic pigment used in paints.”

Mr Kavka added that the substance would not negatively impact the environment. The police found that the group, meanwhile, had committed no crime; the red paint splattered on the site was washed off, and no charges, iDnes reported, were being filed. For the most of the afternoon, after the incident tourists and many passers-by stopped to take videos or pictures of the blood-red water, some of them paddling through it in row boats close to the octopus installation nearby.

Mánes organisers have fielded criticism before: for allegedly failing to communicate with the art community, for the administrative board dealing in a non-transparent manner and more. On Monday, Bolt958 published photos of its members with buckets, writing that Mánes, after having apparently bled out, was now “beginning to smell”. And made clear the protest move was meant to provoke debate. The head of the administrative board, Petr Kuthan, summed up the situation and responded by saying he had no problem talking to members of the group.

“The building was not damaged. The caretaker we have reacted quickly and whitewashed areas splashed with the red colour. I have spoken with members of the group before but have not heard from them since. If they want to contact me, I have no problem with that.”

Public protests by artists or art groups have something of a tradition in the Czech Republic: two years ago members of the group Ztohoven made international headlines when they replaced a flag above Prague Castle with giant red underpants, while artist David Černý floated a giant middle finger on the river two years earlier. Both represented messages for the current Czech president.